This invention relates to a valve which controls the flow rate of a fluid in proportion with the output of a drive source, and more particularly to a proportional flow control valve which is arranged in a by-pass path provided near the throttle valve of the suction pipe in an internal combustion engine.
An engine of electronic control fuel injection type is well know in the art in which a by-pass path is provided near the throttle valve of the suction pipe, and the by-pass path is opened or closed to control the quantity of air sucked into the engine.
FIG. 5 shows a sectional view of one example of a proportional flow control valve used to open and close the above-described by-pass path.
In FIG. 5, reference numeral 1 designates a solenoid unit; and 2, a stationary iron core extended in the solenoid unit axially. A cylindrical case 3 is fitted in the housing of the solenoid unit in such a manner that it covers the inner cylindrical surface of the housing. Flow adjusting means, namely, a movable iron core 4 is disposed in the cylindrical case 3 in such a manner that it is confronted with the stationary iron core 2. The stationary iron core 2 and the movable iron core 4 are disposed inside a pipe 7 with a return spring 5 therebetween. A bobbin 6a on which an electromagnetic coil 6 is wound is put on the pipe 7. An insulating material 3a is inserted between the electro-magnetic coil 6 and the housing of the solenoid unit 1. A spring holder 5a is used to hold the return spring 5 between the stationary iron core 2 and the movable core 4. That is, the movable iron core 4 is urged by the return spring 5 so as to move against the force of attraction of the electro-magnetic coil 6; more specifically, the movable iron core 4 is maintained pushed to the left in FIG. 5 at all times. The electro-magnetic coil 6 is connected to a lead wire 6b extended outside of the solenoid unit.
A spring 8 is arranged in such a manner as to confront through the movable iron core 4 with the return spring 5. More specifically, the spring 8 is disposed between a spring holder 9 and a step 4a formed at the end portion (the left end portion in FIG. 5) of the movable iron core 4 and the spring holder 9 is secured to the end of an adjusting screw 10.
The front end portion of the movable iron core including the step 4a, the spring 8, the spring holder 9, and the adjusting screw 10 are positioned in a chamber 11b formed in a proportional flow control valve body 11 connected to the solenoid unit 1. The proportional flow control valve body 11 has a fluid inlet passageway 11a formed in its end portion near the solenoid unit 1, and a fluid outlet passageway 11c formed in the other end portion (the left end portion in FIG. 5) in such a manner as to communicate with the aforementioned chamber 11b. One end portion (the left end portion in FIG. 5) of the pipe 7 is supported through a wadding 12 by a supporting member 13 which is fixedly provided in the proportional flow control valve body 11, so that, in the proportional flow control valve body 11, the chamber 11b of the latter 11 is separated from the fluid inlet passageway 11a.
The aforementioned adjusting screw 10 is screwed into the left end portion of the proportional flow control valve body 11, where the fluid outlet passageway 11c is formed as was described before; that is, the adjusting screw 10 is extended towards the movable iron core 4. The movable iron core 4 is maintained urged to the right in FIG. 5 by the spring 8 which is supported by the spring holder secured to the adjusting screw 10; that is, the movable iron core 4 is urged in the direction of electro-magnetic attraction at all times.
Fluid holes 7a having predetermined dimensions are formed in the front end portion (the left end portion) of the above-described pipe 7 in such a manner that they are communicated with the fluid inlet passageway 11a of the proportional flow control valve body 11, and that they are closed by the outer cylindrical wall of the movable iron core 4 when the electro-magnetic coil 6 is deenergized.
The movable iron core 4 has a communication hole 4b along its central axis so that the pressure in the chamber 11b communicated with the fluid outlet passageway 11c is balanced with the pressure in the chamber which is defined by the movable iron core 4 and the stationary iron core 2.
The elastic force of the above-described spring 8, which is connected between the step 4a of the movable iron core 4 and the spring holder 9 to urge the movable iron core 4 in the direction of attraction of the electromagnetic coil 6, is adjusted with the adjusting screw 10 so that the position of the movable iron core 4 is determined in advance.
When, in the proportional flow control valve thus constructed, the electro-magnetic coil 6 is energized, the movable iron core 4 is moved towards the stationary iron core 2 against the elastic force of the return spring 5. In this operation, the spring 8 acting in the direction of attraction is extended as the movable iron core 4 moves in the above-described manner.
As the movable iron core 4 is moved towards the stationary iron core 2, the fluid holes 7a of the pipe 7 are opened. As a result, the fluid (air) flowing into the by-pass path from the suction pipe is allowed to flow through the fluid inlet passageway 11a, the fluid holes 7a of the pipe 7, the pitched gaps of the spring 8, and the chamber 11b into the fluid outlet passageway 11c, and through the by-pass path into the suction pipe, thus meeting the fluid in the suction pipe.
In the case of a proportional flow control valve of this type, in order to reduce the hysteresis loss due to the frictional resistance of sliding parts thereof, in general the following duty control method or dither control method are employed. In the duty control method, the electro-magnetic coil is energized and deenergized at a predetermined frequency, and the ratio of the period of time of energization ("on" time) to the period of time of deenergization ("off" time) is changed so that the movable iron core is slightly vibrated. In the dither control method, a predetermined current value is varied (a DC component is modified with an AC component), so that the movable iron core is slightly vibrated.
The proportional flow control valve shown in FIG. 5 is a spool type valve. Therefore, the fluid will leak through the gap between the movable iron core 4 which is in the form of a spool and the pipe 7 which is in the form of a sleeve. That is, as shown in FIG. 6, even when the control duty is set to zero, the flow rate will not be zeroed. On the other hand, in the by-pass air flow control of an internal combustion engine, it is desirable that, during idling; i.e., when it is unnecessary to supply by-pass air, the number of revolutions per minute of the engine is decreased with the quantity of fluid leaking from the by-pass path zeroed so that the fuel is economically used. In a vehicle small in displacement, the absolute value of the flow rate of air necessary for the idling of the engine is small, and therefore sometimes the idling of the engine can be maintained only with the quantity of air leaking from the throttle value. In this case, in view of the relation between the grade and fuel consumption of the vehicle, it is essential to decrease the speed of rotation of the engine as much as possible with the quantity of air leaking from the by-pass path during idling thereby to reduce the rate of fuel consumption.
A spool type valve in which a stopper is provided in such a manner that it abuts against the end of the control spool (movable iron core) when the electromagnetic coil is deenergized has been proposed in the art (Unexamined Japanese Utility Model Publication No. 145078/1988). The stopper serves as a valve seat in the proportional flow control valve as shown in FIG. 5, thus improving the sealing characteristic thereof. However, the provision of the valve seat suffers from the following difficulties: In the proportional flow control valve of this type, in order to reduce the hysteresis loss due to the frictional resistance of the sliding parts, the duty control or dither control is generally employed in which the electro-magnetic coil is energized and deenergized at a predetermined frequency as was described before. Therefore, flow control means comprising the movable iron core will strike
against the valve seat whenever it arrives at or leaves from it, thus producing hammering sounds. As a result, an abnormal phenomenon occurs with the flow characteristic as shown in FIG. 7. The production of hammering sounds and the occurrence of abnormal phenomenon may be eliminated by providing an elastic member of rubber or the like on the striking surface of the flow control means or the valve seat. In this case, the elastic member must be considerably small in hardness so as to sufficiently serve as a shock absorber. However, this method causes another problem that the position of abutment and the load of abutment of the flow control means the valve seat are fluctuated. This fluctuation is significant when the valve is opened.
The same thing can be said about a poppet valve. In the case of a poppet valve, because of its structure, its sealing characteristic can be improved relatively readily; however, the above-described hammering sounds and abnormal phenomenon are liable to take place.
In order to eliminate the above-described difficulties, Unexamined Japanese Patent Publication No. 243582/1988 has disclosed a poppet valve which is designed as follows: A valve body of hard resin with a bellows aside is loosely fitted into a moving core (movable iron core) held with a leaf spring. Displacement of the valve body towards the valve seat is limited with a locking piece. An elastic member is arranged between the valve body and the movable iron core, so that the valve body is pushed towards the locking piece with a predetermined force.
The conventional spool type valve may be employed for manufacturing a proportional flow control valve for opening and closing a path adapted to by-pass the throttle valve of an internal combustion engine. However, the valve thus formed suffers from the following difficulties: As was described above, fluid will leak through the gap between the spool, namely, the movable iron core and the sleeve (cylindrical member). Hence, in the region of operation in which the flow rate should be zeroed, it is impossible to completely zero the flow rate. In the case where the valve seat is provided, in the duty control or in the dither control hammering sounds are produced or an abnormal phenomenon occurs when the flow control means reaches or leaves the valve seat. The production of hammering sounds and the occurrence of abnormal phenomenon may be eliminated by providing an elastic member on the striking surface of the flow control means or the valve seat. However, this method causes another problem that the position of abutment and the load of abutment of the flow control means the valve seat are fluctuated.
One example of the poppet valve has tried to solve the above-described problems. In the example, similarly as in the above-described poppet valve, a valve body of hard resin with a bellows is loosely fitted into a movable iron core held with a leaf spring, and it is pushed towards a locking piece by an elastic member. However, the poppet valve is still disadvantageous in the following points: In the poppet valve, it is necessary to determine its operating characteristic from the balance of the force provided by at least two kinds of coil springs, the spring, and the bellows in combination and the force of attraction of the electromagnetic coil. Hence, the determination of the operating characteristic is rather difficult, and is liable to be fluctuated; that is, adjustment of the poppet valve is troublesome. Furthermore, in the above-described example, it is essential to communicate the gap between the valve body and the holding plate, the gap between the holding plate and the magnet plate, and the gap between the movable iron core and the stationary iron core with one another so that the pressures therein be equal to one another; otherwise, a damper effect takes place, thus making it impossible to operate the poppet valve at high speed. Thus, the poppet valve requires a plurality of communicating spaces; in other words, the probability is increased as much that the communicating spaces are clogged up by foreign matters. When the communicating space or spaces are clogged up, the flow characteristic is no longer correct.
If, in the control of the flow rate of by-pass air in an internal combustion engine, a blow-by gas flows to the valve from upstream of the suction passageway or a gas spitted by the engine flows thereto from downstream, then carbon or oil contained in the gas deposits on the sliding surfaces of the valve and the movable iron core in the proportional flow rate control valve, thus adversely affects the slide characteristic of the valve. As a result, the following problems are involved: That is, when the movable iron core is stuck to the valve, the repulsive force cannot be absorbed which may be induced when the valve strikes against the valve seat. As a result, hammering sounds are produced, and the flow characteristic becomes unstable. In the case where the sliding operation is obstructed with the elastic member compressed to some extent, the valve is left opened even if the drive source is turned off; and at worst the engine is rotated excessively; that is, a so-called "engine runaway" takes place.
A valve assembly may be provided which can eliminate the above-described difficulties. In the valve assembly, when the valve (body) moves a predetermined distance after leaving the valve seat, the flow rate is determined from the minimum flow area between the Valve and the valve seat. On the other hand, the valve assembly is so designed that a predetermined striking part is provided for the valve seat with the inclination permitted by the clearance in the sliding region. Therefore, if the gap between the valve seat and the holder (or locking piece) is so small, then the annular gap between the valve seat and the holder is varied irregularly. Thus, the flow characteristic becomes wavy as shown in FIG. 8. Furthermore, if, in the case where the stroke of the valve assembly is increased in order to increase the maximum flow rate thereof, the gap between the valve seat and the holder is excessively small as was described above, then the following problem is involved: That is, even if the stroke of the valve assembly is increased, the small gap between the valve seat and the holder serves as a choke means to limit the flow rate. This difficulty may be eliminated by increasing the inside diameter of the valve seat; however, the increasing of the inside diameter of the valve seat arises another problem that when the diameter of abutment of the valve seat and the valve body is increased, the valve assembly is liable to be non-uniformly pressured, and in order to eliminate this difficulty, it is necessary to increase the diameter of the slide support of the movable iron core, and the resultant proportional flow control valve is unavoidably bulky. In addition, when the diameter of abutment of the valve body and the valve seat is increased as was described above, then the valve assembly is increased in flow gain, and therefore it is lowered in control characteristic.
In addition, since the maximum flow rate is controlled by engagement of the stopper press-fitted in the movable iron core with the tapered end portion of the stationary iron core, hammering sounds are produced by the stopper and the stationary iron core before the valve is fully opened, and the flow characteristic is adversely affected. In order to eliminate this difficulty, the following method has been proposed: even when the supply voltage is maximum, by balancing the electromagnetic force Of attraction with the force of repulsion of the springs, the stroke is made maximum before the stopper strikes engages with the stationary iron core. However, the method is disadvantageous in that, since the fluctuations in the force of attraction and in the elastic force affect the maximum stroke, whereby the maximum flow rate is fluctuated.